Japanese Unexamined Patent Application Publication No. 2000-114039 discloses a switching apparatus that is a solenoid driving apparatus configured to detect a load current passing through a solenoid serving as load and to control a switching element in PWM manner so as to minimize a deviation of the load current from a control target value supplied from a microcontroller.
Japanese Unexamined Patent Application Publication No. 2004-22947 discloses a function of stopping operation of a control circuit if a power source voltage supplied to the control circuit becomes lower than a predetermined value, thereby preventing a malfunction of the control circuit in a switching apparatus. The function is called UVLO (Undervoltage Lockout) and is known to be included in the above-mentioned solenoid driving apparatus.
With reference to FIGS. 1 and 2, operation of the UVLO function in the switching apparatus according to the related art will be explained. FIGS. 1 and 2 are flowchart and waveform diagram illustrating operation of the switching apparatus according to the related art. VB′ is a power source voltage supplied to the control circuit, Vth′ is the predetermined value to stop the control circuit, Vamin′ is a minimum operation voltage of the control circuit, IL′ is a load current passing through the solenoid, and VG′ is a gate drive signal to control ON/OFF of the switching element.
In step S101, the switching apparatus carries out a normal operation. When the DC voltage VB′ rises, the control circuit supplies power to the solenoid by controlling the switching element in the PWM manner with the gate drive signal VG′ that is a pulse signal alternating H (high) level and L (low) level. When the gate drive signal VG′ is at H-level, the switching element establishes an ON state. In response to the operation of the switching element, the load current IL′ increases and decreases.
In step S102, the control circuit checks to see if the power source voltage VB′ is lower than the predetermined value Vth′. If the power source voltage VB′ is equal to or higher than the predetermined value Vth′, the switching apparatus continues the normal operation in step S101. If the power source voltage VB′ is lower than the predetermined value Vth′, the control circuit stops in step S103.
Namely, at time t1′ illustrated in FIG. 2, an operating state of a peripheral device (not illustrated) causes the DC voltage VB′ to become lower than the predetermined value Vth′ and the control circuit stops the switching element by setting the gate drive signal VG′ at L-level.
In step S104, the control circuit checks to see if the power source voltage VB′ is equal to or higher than the predetermined value Vth′. If the power source voltage VB′ is equal to or higher than the predetermined value Vth′, the control circuit restarts the switching element in step S105. If the power source voltage VB′ is lower than the predetermined value Vth′, the control circuit continuously makes the switching element inoperative in step S103.
Namely, in the period from time t1′ to time t2′ illustrated in FIG. 2 in which the DC voltage VB′ is lower than the predetermined value Vth′, the control circuit keeps stopping the switching element by maintaining the gate drive signal VG′ at L-level. Thereafter, at time t2′, the DC voltage VB′ becomes higher than the predetermined value Vth′ and the control circuit restarts the PWM control of the switching element by outputting the gate drive signal VG′. When the switching element restarts, the load current IL′ starts to increase and decrease.
In step S106, the switching apparatus carries out the normal operation, like in step S101.
The switching apparatus with UVLO function according to the related art repeats the above-mentioned operation, thereby preventing a malfunction of the control circuit when the DC voltage VB′ drops.